In conventional devices which communicate over telephone lines, most of the telephone line interface circuitry is connected to the telephone line by a mechanical switchhook. In this manner, the interface circuitry is not coupled to the line when the device is "on-hook" and the switchhook is open-circuited. Such an arrangement protects the devices from damage from noise bursts on the telephone line such as are caused by lightning strikes or other electrical surges. However, devices which receive incoming calls must monitor the line at all times to detect ringing. Consequently, a ring detector circuit is conventionally directly coupled to the line. In addition, a "snubber" circuit, comprising a resistor in series with a capacitor, is frequently connected across the switchhook in order to protect the switchhook contacts from destructive arcing.
In an ordinary telephone, the switchhook contacts are closed to connect the telephone to the line when the handset is lifted from its cradle. In many electronic devices, the device closes the switchhook contacts in response to the detection of incoming ringing by the ring detector or automatically closes the contacts when the device wishes to contact another location over the line.
Problems are created when two or more electronic devices are connected to the same telephone line or when an electronic device is connected together with an ordinary telephone to a single line. In these situations, the ring detector cannot be designed so that the device "answers" the telephone every time ringing is detected, since the incoming call may not be intended for that device. Therefore, these devices generally include time delays or are designed only to make outgoing calls and not to answer any incoming calls. However, even devices which make only outgoing calls must monitor the telephone line to determine when the line is in use or incoming ringing is present. In the former case seizure of the telephone line would interrupt use of the line by the established party. In the latter case a connection will be established with a calling party even though nothing has answered the phone. Consequently, such devices must incorporate a ring detector.
An example of an electronic device which automatically initiates calls is a sensing device such as that disclosed in application U.S. Pat. No. 06/907,120 entitled RESIDENTIAL FUEL OIL SENSOR, filed on Sept. 12, 1986, now U.S. Pat. No. 4,845,486 by John D. Knight and assigned to the same assignee as the present invention which application is hereby incorporated by reference. In the disclosed system, a sensor located in a residential home initiates a telephone call to a central location when the level of oil in the residential tank is low. Since it is usually not economically feasible to provide a dedicated telephone line for such a sensor, the sensor is normally connected to the existing residential telephone line. Accordingly, the sensor incorporates a ring detector to monitor the line so that the sensor will not initiate a call if the line is in use or if ringing is present on the line.
A typical prior art ring detector comprises several electronic components which together constitute a substantial portion of the cost of the sensor. In a situation like that disclosed in the aforementioned application, where a large number of sensor devices must be used to implement a total system, conventional ring detectors add a substantial cost to the alarm system.
Such sensor devices must typically contain circuitry to receive and process electronic information received from the central location, but the electronic information is usually transmitted and received over the normal telephone communication bandwidth (300 Hz-3000 Hz). Thus the exisitng circuitry cannot be used to detect ringing and dial pulses which are normally outside the communication bandwidth.
Therefore, it is an object of the present invention to provide an improved ring signal detector.
It is an another object of the present invention to provide a simple and inexpensive means for detecting ring signals.
It is a further object of the present invention to provide a ring detector which can also detect a pulse dial signal on a telephone line.
It is yet another object of the present invention to provide improved immunity to noise on the telephone line.